Intensification of land use, as well as the intensification of industrialization and farming on a global scale, have affected the natural cycle of various nutrients found in surface waters. While different pollutants are affecting the quality of water resources, phosphorus is one that is attracting more and more attention. This substance, which is present in deoxyribonucleic acid (DNA) and several intermediate metabolites, is essential to all forms of life. Phosphorus is very dynamic, capable of being transformed biologically and chemically through a series of processes which constitute the phosphorus cycle. Phosphorus can be found in particulate, organic or soluble form. The soluble form of phosphorus includes orthophosphates and polyphosphates. The orthophosphate form, often called phosphate, draws particular attention because it is the only form directly assimilated by bacteria, plants and algae. Polyphosphates, which are compounds having two or more phosphorus molecules, will decompose more or less rapidly into orthophosphate by hydrolysis. The term “total phosphorus” includes all forms of phosphorus in water and is used as the term of reference term in Rules & Regulations presently in effect. Phosphorus exists naturally and is exploited by man to satisfy industrial and agricultural needs. Phosphorus uses are numerous, but agriculture alone consumes about 97% of world production. The phosphorus found in surface water comes mainly from surface intake, mostly anthropogenic sources, mainly shared between urban, industrial and agricultural sectors. Since 1985-1990, phosphorus removal processes from wastewater treatment plants and the use of detergents less rich in phosphorus have helped reduce the proportion of urban and industrial phosphorus, in favor of phosphorus of agricultural origin. The discharge of wastewater of urban origin is the largest point source of surface water phosphorus. The phosphorus in wastewater of municipal origin comes essentially from human metabolic waste (urine, faeces), and various household detergents, among others-those used in dishwashers. Considering that the average quantity of waste water produced daily by individuals is around 320 L/d, the average concentration of total phosphorus in wastewater of municipal origin would be approximately 5 mg Ptot/L. If it is assumed that 2-3% of the organic solids are made up of phosphorus, then a typical waste water containing 20 mg/L of total suspended solids (TSS) contains about 0.5 mg/L of particulate phosphorus, and the balance, about 4.5 mg/L, is therefore composed of soluble phosphorus or orthophosphate and polyphosphate.
In the presence of nitrogen and carbon, an excessive amount of phosphorus can lead to eutrophication of a lake. This phenomenon is defined by nutrient enrichment associated with the natural aging of a body of water, leading to a series of harmful consequences to the extent that this enrichment is accelerated by human activities. Phosphorus is especially significant because it is usually the limiting factor for eutrophication in natural environments of fresh water, which means that the concentration of phosphorus will dictate the impact on the ecosystem, even if other elements are found in abundance. The consequences of eutrophication of a body of water are numerous. They are characterized among others by increase in the growth of plants and algae, increased bacterial biomass, the occurrence of undesirable species (cyanobacteria), a reduction in water transparency that deprives the column of light, a decrease in dissolved oxygen, an increase in pH, taste and odor problems affecting the production of drinking water, as well as multiple nuisances for water activities. It is known that eutrophication can even lead to a decline in freshwater biodiversity. Indeed, the oxygen deficits induced by high bacterial concentrations and toxins released by the masses of algae degradation can cause death in certain species of fish and make water unsafe for birds. It is also possible that undesirable species, better adapted to the new conditions, take the place of native species. Moreover, the aesthetic quality of the water bodies is reduced by the decomposition of the biomass formed by the algae, which involves a decrease in the depth of lakes through increased sedimentation, foam generation on the surface of the water, the proliferation of harmful insects, as well as the release of bad odors. It was highlighted that a surplus of phosphorus was the main cause of outbreaks of blue-green algae, resulting from the excessive proliferation of cyanobacteria, strongly coloring the water with green, turquoise or red tint. These microscopic entities, which possess both the characteristics of bacteria and algae, produce toxins which present a serious risk to the health of users of aquatic environments. Moreover, the surfaces of bodies of water, used for drinking water supply sources, can also be affected by the massive outbreak of blue-green algae, causing serious problems for affected drinking water production stations. Thus it is desirable to minimize phosphorus inputs from wastewater discharges, particularly through the use of phosphate removal equipment in municipal wastewater treatment stations flowing into a lake. There are several methods of phosphorus removal which are widely used worldwide. In all cases, these methods involve fixing the phosphate ions in or on solids which are then separated from the effluent, as reported by de-Bashan et al. in “Recent advances in removing phosphorus from wastewater and its future use as fertilizer (1997-2003)” Water Research 38 (19): 4222-4246 (2004). Depending on the process, these solids can take the form of a precipitate of an insoluble metal salt, substances having high chemical affinity for the phosphorus, a microbial biomass or a plant biomass.
Phosphorus is essential to all forms of life. Therefore, it is possible, under certain conditions, to carry out phosphate removal of wastewater by certain biological agents, either bacteria or plants. Since the 1970s, increasing knowledge of biological mechanisms led to the conception of various types of “activated sludge” processes for performing phosphorus removal up to concentrations as low as 0.1 mg/L in the effluent. This phosphate removal is performed by phosphate accumulating bacteria (Bio-P) that are able to accumulate up to 10% of their mass in phosphorus under certain conditions, compared with about 2% for the bacteria used in conventional activated sludge systems. Generally, as described by Tchobanoglous et al. in “Wastewater Engineering: Treatment and Reuse” Metcalf & Eddy, McGraw Hill Series in Civil and Environmental Engineering, 4th edition, McGraw-Hill, New-York (2003), the biological phosphorus removal mechanism is based on the passage of the Bio-P bacteria in an anaerobic reactor sequence, with no oxygen or nitrate, and anoxic or aerobic reactor, containing oxygen or nitrates, where the removal of nitrogen is generally made jointly to phosphorus removal. Most bio-P bacteria will be evacuated with sludge from a secondary settling tank, taking with them the phosphorus they have drawn from the wastewater, and sludge is recirculated to ensure a continuous supply of Bio-P bacteria in the system.
If biological phosphate removal of wastewater has greatly been studied in the biomass suspension systems, it has been much less in fixed environment systems. In this type of system, a biofilm composed of biomass grows on a fixed support. Biofilters on organic bed, such as those described in U.S. Pat. No. 5,206,206, U.S. Pat. No. 6,100,081 and U.S. Pat. No. 7,374,683, allow the implementation of this type of biological process. The process described in U.S. Pat. No. 5,206,206 uses a mixture of peat and an iron compound in order to capture more phosphorus. The processes described in U.S. Pat. No. 6,100,081 and U.S. Pat. No. 7,374,683 disclose blends of peat and wood chips, the latter acting as a structuring agent and diffuser for water and air, but do not have a specific capacity for purifying phosphorus. The difficulty to expose the fixed biomass to alternating anaerobic and aerobic environments, essential for Bio-P bacteria, constitutes the major problem for biological phosphorus removal in a fixed environment. The majority of wastewater treatment plants that practice phosphorus removal use processes that involve the injection of chemicals in order to precipitate phosphates, as reported by de-Bashan et al. supra, the precipitate being subsequently separated from the effluent by decantation or by filtration. The main features to consider for chemical phosphorus removal are: which chemicals are used, the required dosage, the injection site, the equipment required and the sludge produced. In most cases, trivalent metal salts are used. These products, commonly distributed in liquid form, are aluminum sulfate (Al2(SO4)3*7H2O) (also known as alum), iron chloride (FeCl3) and iron sulfate (Fe2(SO4)3*9H2O). Quicklime (CaO or Ca(OH)2), formerly widely used because of its low cost, is now much less used because of the much higher sludge mass produced, as well as various issues related to operations and handling of the reagent. Iron chloride (FeCl3) was used in the work carried out by Eberhardt et al. described in “Phosphate removal by refined aspen wood fiber treated with carboxymethyl cellulose and ferrous chloride.” Bioresource Technology 97: 2371-2376. (2005). Other related works are described by Eberhardt et al. in “Biosorbents prepared from wood particles treated with anionic polymer and iron salt: Effect on particle size on phosphate adsorption” Bioresource Technology 99:5617-5625 (2008), by Han et al. in “Removal of phosphorus using chemically modified lignocellulosic materials” 6th Inter-Regional Conference on Environment-Water—Land and Water Use Planning and Management—Albacete, Spain, Sep. 3-5, 2003 and by Kuo et al. in “Sorption and desorption of chromate by wood shavings impregnated with iron or aluminum oxide” Bioresource Technology 99(13): 5617-5625 (2008), by Robertson in “Treatment of Wastewater Phosphate by Reductive Dissolution of Iron” Journal of Environ. Qual. Vol. 29, September-October 2000 pp. 1678-1685, and by Robertson et al. in <<Treatment of Wastewater Phosphate by Reductive Dissolution of Iron>> Journal of Environ. Qual. Vol. 40 (2011) pp. 1955-1962.
In contexts where the flow rates to be treated are minor, the phosphorus removal techniques presented above are more or less applicable, particularly because of the significant costs related to the acquisition of equipment required and the complexity of operations involved.
To meet the needs of small installations with limited budget and manpower, several studies have focused on phosphate removal by passive capture filters. The general principle of the passive capture of accumulating a problematic substance within a filter consists of materials having a high chemical affinity for the contaminant, so that retention of the contaminant occurs by its phase change from liquid to solid. The filter media will be able to capture the pollutant until saturated, after which it must be replaced or regenerated. To limit substitutions, as noted by Cucarella et al. in <<Phosphorus Sorption Capacity of Filter Materials Used for On-site Wastewater Treatment Determined in Batch Experiments-A Comparative Study>> Journal of Environmental Quality 38(2): 381-392 (2009), it is important that the filter media has a strong capacity to support the pollutant, to consider the physical properties of the filter media, and in particular its specific surface determined by particle shape, size and porosity. The property inherent to pollutant management is generally designated as “sorption” and involves a combination of adsorption, ion exchange and precipitation. Phosphorus capture mechanisms by iron (Fe) and aluminum (Al) are quite similar, whereas the action of calcium (Ca) is different. Forms of Fe and Al having the greatest affinity for phosphorus are oxyhydroxides, also called sesquioxide metals, consisting of a compact arrangement of oxygen and/or hydroxyl (OH—) ions, which contain metal ions in their octahedral cavity, iron hydroxide (Fe(OH)3) and aluminum oxide (Al2O3) being examples. Sesquioxides can be crystalline or amorphous structures. Therefore, without any particular organization, the latter can support many more orthophosphate ions than the crystalline forms. For nearly two decades, numerous studies have focused on materials with high concentrations of Fe, Al and Ca with phosphorus sorption capacity (PSC), which may constitute the packing of passive capture filters, as reported by Johansson Westholm <<Substrates for phosphorus removal-Potential benefits for on-site wastewater treatment?>> Water Research 40(1): 23-36 (2006) and by Cucarella et al., cited above, these materials including natural products, industrial byproducts and manufactured goods. Natural products, generally less expensive than industrial byproducts and manufactured goods, can be used directly or undergo minor treatments such as crushing or heating. Among the materials contained in this category are various sands and gravels, apatite, opoka, wollastonite, zeolites, peat, mollusk shells, bauxite, dolomite, alunite, limestone and polonite, as listed by Vohla et al. in “Filter materials for phosphorus removal from wastewater in treatment wetlands—A review” Ecological Engineering 37(1): 70-89 (2011).